The requirements imposed on sawing tools are particularly high, for example, in semiconductor technology. In this field, brittle and hard workpieces, for example rods or blocks of semiconductor materials such as silicon or of compound semiconductor materials such as gallium arsenide, are sawn into thin wafers. The wafers having a thickness of 0.1 mm to 1 mm are used as starting products for producing a wide variety of electronic components or, alternatively, for producing solar cells.
A low cutting loss and a high geometrical quality of the sawn-off wafers, associated with high sawing rates, are the most important requirements to be fulfilled for an economical sawing process.
Annular saws, wire saws and ribbon saws are the most frequently used sawing machines when it is a matter of cutting brittle and hard workpieces with high precision. Hitherto, annular saws, in particular, have been used to saw rods of silicon single crystals into wafers. However, the saw blades of annular saws are no longer able to follow the unbroken trend toward single crystals with ever larger diameters--the technology has now mastered the pulling of crystals having diameters of 300 mm in the required quality--without disadvantages. It is only by means of an increase in thickness that fairly large saw blades can be stabilized in such a way that wafers having the required geometrical quality are obtained. With the unavoidably wider cutting edges, higher cutting losses have to be accepted. The cost effectiveness of wafer production is critically impaired particularly if the lost material is expensively produced, high-quality semiconductor material. A further disadvantage arises for the sawing process out of the cost of designing the machine and the sawing tool which increases with the workpiece diameter.
Wire and ribbon saws with fixed grinding grain form an alternative to annular saws, particularly for workpieces having large diameters. For smaller workpieces, it is also possible to use wire saws with loose lapping grain, the wire being repeatedly coiled around deflection rollers so that a large number of wafers can be produced at the same time. As the length of the cutting slit increases, however, problems increasingly arise which relate to the conveyance of lapping grain to the cutting point. The achievable cut accuracy is also thereby limited. Ribbon saw blades for sawing hard and brittle material are, as a rule, provided along one edge with cutting platelets or continuously with a cutting coating out of which the fixed diamond or boron nitride cutting grains project. The ribbons are welded at their ends to form ribbon loops. Such ribbon loops are fed in a circle over deflection rollers. The feed movement of the workpiece towards the cutting edge takes place perpendicularly to the direction of movement of the ribbon.
The rotational speeds of wire or ribbon loops with fixed cutting grains reach, depending on the material, appreciable values. In sawing hard and brittle semiconductor materials, such as for example silicon, these are preferably between 20 m/s and 120 m/s. Since extreme trueness of cutting line is required of the cutting tools, the loops have to be tensioned with a high tensile force in order that the lateral deflection of the saw blade as a consequence of the effect of the sawing forces remains in a tolerance range of a few Mm. In addition, owing to the constant change in direction during rotation, a high flexural fatigue strength of the loop material is necessary. Particularly suitable materials for saw ribbons and wires are steels and special alloys because of their high tensile strength and elastic limits. However, it has hitherto not been possible to make optimum use of the beneficial material properties. The weak points of wire and ribbon loops are formed by the seams at which the wire or ribbon ends have been welded or soldered to form loops. Allowance has to be made for the lower strength of the seam point by adjusting the ribbon tension. If, as in the case of machining silicon, cutting with the required quality is only possible with high tensile loading of the sawing tool, the ribbon cross section and wire diameter have to be increased correspondingly. In addition, seam stresses are produced which additionally impair the flatness and the concentric running of the ribbon loops and adversely affect the precision of the cuts.
GB 2,055,643 A discloses the fact that a seamless ribbon can be produced from a flat, thin disk by stretching the inner regions. This method is, however, usable only to a limited extent as a result of the size of the available disk material and results in ribbons having a very nonuniform degree of deformation and varying stress and strength distribution.
An object of the invention is therefore to provide a method for producing precise ribbon and wire loops of any required size and with a high-load carrying capacity which have as constant as possible a strength their entire circumference. A further object of the invention is to provide a device for producing such ribbon and wire loops.